1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images to toner images, which toner has excellent anti-offset performance, fluidity and durability, a method for forming toner images using the toner, a method for producing the toner, and a toner container for containing therein the toner.
2. Discussion of Background
In electrophotography, a latent electrostatic image is formed on a photoconductor comprising a photo-conductive material, using various means, and the formed electrostatic image is then developed with a toner to a visible toner image, and when necessary, the developed visible toner image is then transferred to a sheet of paper or the like, and fixed thereto with the application of heat and/or pressure thereto, or using a vapor of a solvent, whereby a hard copy can be obtained.
As methods for developing the electrostatic image to the visible image, using the toner, there are known, for instance, magnetic brush development method, cascade development method, and powder cloud development method. In any of these development methods, the step of fixing the toner image to a transfer sheet is a very important step. In particular, when the toner image is thermally fixed, using heat rollers, the toner image in a fused state comes into contact with the surface of the heat rollers, so that a phenomenon that the toner image adheres to the surface of the heat rollers and is transferred thereto, that is, a so-called offset phenomenon, may take place. It is required that such offset phenomenon should be prevented from occurring.
Conventionally, in order to prevent the occurrence of the offset phenomenon, the surface of an image fixing roller is made of a material having excellent releasability with respect to the toner, such as silicone rubber or fluorine-based resin.
Furthermore, conventionally, in order to prevent the occurrence of the offset phenomenon more effectively, and also to prevent the fatigue of the surface of the image fixing roller, the surface of the image fixing roller is covered or coated with a thin layer of a liquid which has high releasability with respect to the toner, such as silicone oil or a fluorine-based oil, which may be referred to as the offset preventing liquid.
The above-mentioned method is extremely effective for preventing the occurrence of the offset phenomenon, but has a problem that an apparatus for supplying the offset preventing liquid to the image fixing roller is necessary, which makes the image fixing apparatus complicated in mechanism. Furthermore, the step of coating the surface of the image fixing roller with the oil has the risk of bringing about the problems that the surface layers of the image fixing roller are separated and accordingly the life of the image fixing roller is significantly shortened while in use.
Instead of using such an oil supplying apparatus, there has been proposed a method of supplying an offset preventing liquid from toner particles themselves in the course of the image fixing step with the application of heat and/or pressure thereto. More specifically, a releasing agent such as a low-molecular-weight polyethylene, or a low-molecular-weight polypropylene, is added to the toner particles and is caused to ooze from the toner particles at the image fixing step while heat and/or pressure is applied thereto as disclosed, for instance, in Japanese Laid-Open Patent Application 60-230663 and Japanese Laid-Open Patent Application 1-234858.
Such a toner that includes the releasing agent in the toner particles thereof is generally produced by uniformly mixing a binder resin, a coloring agent and the releasing agent, each in a powder-like state, fusing and kneading the mixture, pulverizing the kneaded mixture to prepare a pulverized material and classifying the pulverized material.
However, generally the releasing agent has a lower molecular weight and a less hardness than the binder resin has, so that at the above-mentioned pulverizing step, the pulverizing of the kneaded mixture takes place selectively either inside the releasing agent or at the interface between the releasing agent and the binder resin. As a result, the releasing agent tends to be exposed at the surface of the toner particles. The larger the particle size of the releasing agent in the kneaded mixture of the toner, the more conspicuously the above-mentioned tendency is developed due to the non-uniformity in the mechanical strength of the components in the kneaded mixture, while the smaller the particle size of the releasing agent in the kneaded mixture of the toner, the less the tendency is developed due to the increased uniformity in the mechanical strength of the components in the kneaded mixture. Such a releasing agent tends to be subjected to excessive pulverizing, so that that the particle size of the releasing agent tends to be more considerably decreased than that of the binder resin.
It is known that as the amount of the releasing agent exposed on the surface of the toner particles is increased, the releasing agent is released from the surface of the toner particles and adheres to the surface of carrier particles or a charging member, so that a so-called spent phenomenon takes place by which the charging performance of the carrier particles and that of the charging member are significantly lowered, as a result, the durability of the toner is markedly lowered.
It is also known that the presence of the above-mentioned releasing agent within the toner impairs the fluidity of the toner and/or the transfer performance of the toner to a transfer sheet.
The pulverizing of the toner particles at the interface between the binder resin and the releasing agent takes place not only in the course of the production of the toner, but also within a development unit in the course of the stirring of the toner. When such pulverizing takes place, the toner aggregates or the fluidity of the toner is lowered, so that proper development and image transfer cannot be carried out and image quality obtained is lowered.
In order to solve the above problem, the following proposals have been made, in which the state of the added releasing agent in the toner is controlled:
For example, in Japanese Laid-Open Patent Application 3-243956, there is proposed the use of a toner having a 200 to 5000 .ANG. average lattice length of a primary peak of a small angle X-ray scattering (SAXS); in Japanese Laid-Open Patent Application 3-296067, there is proposed a toner at a cross section of which there are formed island-sea structures composed of a binder polymer and polypropylene, with a maximum diameter in a major axis of island portions formed by the polypropylene being in a range of 200 to 3000 .ANG., and an average interval among the islands being 1 .mu.m or less; and in Japanese Laid-Open Patent Application 5-45925, there is proposed a toner comprising toner particles on the surface of which there is deposited a releasing agent with a particle size of 100 to 5000 .ANG., having a fusing initiation temperature--fusing termination temperature difference of 50.degree. C. or less, and a melting point of 60 to 180.degree. C.
In order to maintain the development performance and abrasion resistance of a photoconductor, there is proposed in Japanese Laid-Open Patent Application 5-197199 a toner on the surface of which there are dispersed polyolefin particles with a particle size of 0.01 to 0.5 .mu.m in an amount of 2 to 20%; in Japanese Laid-Open Patent Application 7-301951 a toner comprising a binder resin and a releasing agent with an SP value difference thereof being 1.5 or less; and in Japanese Laid-Open Patent Application 7-271095 a toner comprising a releasing agent with a crystallization degree in a range of 40 to 60% when the releasing agent is in the toner.
However, in any of the above prior art, the average values with respect to the overall characteristics of the toners are defined, but none of the prior art analyzes the state of the releasing agent in the toner particles with a smaller particle size than the average particle size of the toner particles, which smaller toner particles are in the range of 4 .mu.m or less, and cause problems mainly in the fluidity of the toner and also cause the spent phenomenon. As a matter of course, problems in connection with the fluidity of the toner and the spent phenomenon thereof have not sufficiently been solved by the prior art.
Japanese Laid-Open Patent Application 10-69125 discloses a toner for electrophotography comprising toner particles, each toner particle comprising a binder resin, a coloring agent, and a wax which serves as a releasing agent. In this reference, the content of the releasing agent is defined by the ratio of the content (w) of the wax in the toner particles with a volume mean diameter of 3 .mu.m to the content (W) of the wax in the entire toner particles of the toner with a volume mean diameter of 8 to 12 .mu.m as being 1.00 to 1.20, that is, w/W=1.00 to 1.20. However, this reference neither teaches nor suggests anything about the state of the releasing agent in the toner particles with the particle size in the range of 4 .mu.m or less, which mainly cause the problems with respect to the fluidity of the toner and also with respect to the spent phenomenon. Accordingly the problems are not effectively solved by the invention disclosed in this reference.
This reference does not specifically describe how to produce the toner, and the toners disclosed in the examples thereof cannot be produced even by the methods described in the examples.